This invention deals generally with magnetic brush developing structures and in particular with a developer in which measured quantities of the mix are brought to the developing station.
The technology dealing with design and construction of magnetic brushes is well known. The use of magnetic flux generating means positioned inside rotating cylinders for the purpose of attracting a quantity of granular mass to its surface to form an upstanding brush and applying toner to the image-bearing medium is a well known developing technique in this art.
It is well understood in this technology that the magnetically attractable developer mixes, which are made up of magnetically attractable materials plus an electroscopic resin particle, suffer fatigue due to the degradation of the developer mix. Fatigue is caused by the severe forces present in the conventional developer brush systems. One of the major areas in the magnetic brush construction concerns the use of metering blades and doctoring devices to control the height of the brush at the developing site. This is a critical and important aspect of the known construction since the success of proper magnetic brush development depends on the quality of the brush. The size of the brush, which is brought against the surface of the area to be developed, must be controlled within strict limits to produce quality reproductions. If the brush is too small, development will be inadequate and give poor or low density images. If the brush is too large or too thick, it will disrupt the paper from its assigned path and dislodge any toner that is applied. Hence, it is critical to control the size of the brush.
The prior art systems, as stated earlier, used mechanical means in the path of the developer mix to doctor the height of the mass to a predetermined level, and otherwise compress it in order to achieve the desired height. The use of such doctoring blades and compression devices in the presence of magnetic fields, while they control the height of the brush formation, were detrimental in terms of compacting the resinous materials interfering with its free-flowing quality and ultimately to press the toner particles onto the carrier so as to defeat the triboelectric charges necessary to be generated by the rubbing action between the two dissimilar materials comprising the mix.
These disadvantages have been obviated in the present invention by eliminating the need for doctoring or shearing or compressing the developer mix in order to form a brush of a predetermined height or thickness or otherwise scrape or remove the toner under conditions where it is being held by a high magnetic flux field. The need for any such metering devices has been eliminated by providing a measured mass and controlled amount of developer mix from the supply when it is first delivered to the brush forming developing cylinders. The controlled mass forms into an upstanding brush-like formation under the influence of the magnetic flux generating means within the developer cylinders. It will be appreciated that by limiting and controlling the mass of developer mix delivered to the developer cylinders limits the size of the brush formation that occurs at the developer station under the influence of the high magnetic flux field.